The present invention relates to a method of providing a hydrogen or phosphorus distribution image of an object to be examined by use of a nuclear magnetic resonance (NMR) phenomenon, and more particularly to an imaging method of obtaining separative distribution images for two chemical shift exhibiting substances for a short time.
In an NMR imaging in which spins of atomic nuclei of hydrogen are to be examined, an image representative of the distribution of water contents in a living body and an image representative of the distribution of fat contents therein are separately obtained to increase the quantity of information necessary for diagnosis. There are known some approaches to obtain separative magnetization distribution images for two kinds of substances, in an object to be examined, which exhibit known chemical shifts. Radiology, Vol. 159 (1986), pp. 783-789 discloses a method using a multi-echo sequence which produces first and second echoes. This method includes a measurement based on a first multi-echo sequence in which chemical shift effects for the phases of two kinds of spins are cancelled for each of first and second echoes and a measurement based on a second multi-echo sequence in which a first echo produces a difference of .pi. between the phase shift effects for the two kinds of spins and a second echo produces a difference of 2.pi. therebetween. Separative images are determined from four images which are obtained by such a method. U.S. Pat. No. 4,739,265, which has some common inventors with the inventors of the present application, proposes a method of obtaining two separative images by solving simultaneous equations of the real and imaginary parts of NMR image data acquired through a measurement sequence which produces a difference of .pi./2 or (2n+1).pi./2 (n=0, 1, 2, 3 . . . ) between the phase shift effects for two kinds of spins to be separated from each other.
What is common to the two conventional methods mentioned above is that phase encoding is used for a mapping in a first direction and a read-out gradient is used for a mapping in a second direction. Accordingly, as well known, the measurement sequence must be repeated n times while changing the value of integration of the phase encoding gradient with respect to time in n ways, in order to obtain images having n pixels for the first direction. In the former of the above two conventional methods, since such a consecutive repetition of the measurement sequence is performed for each of the two kinds of multi-echo sequence, there is involved a drawback that an object to be examined is placed under restraint within the NMR apparatus for a long time.